1. Field of the Invention
The present invention relates to a system for melt-spinning a synthetic yarn from a thermoplastic polymer, and more particularly, to a method and an apparatus for obtaining such a yarn having a durable mechanical and thermal stability in practical use, at a low manufacturing cost, by only a single continuous process. This system is suitable for producing for industrial use a yarn composed of thicker individual filaments.
2. Description of the Prior Art
In the conventional method, a thermoplastic polymer is melted, spun from a spinneret, cooled and solidified, and continuously taken up as a package of an undrawn yarn having a low molecular orientation. Thereafter, the undrawn yarn is drawn while heated, by a separate process, to obtain a drawn yarn having a durable mechanical stability in practical use.
Recently, to reduce costs and save energy, many attempts have been made to produce a drawn yarn through a single continuous process following the melt-spinning of the polymer from a spinneret.
A direct spinning system is one such attempt, in which a melt-spinning step is directly connected to a drawing step so that an undrawn yarn spun from a spinneret is continuously introduced, without being taken up as a package, to a group of hot rollers for drawing. This system, however, has a drawback of a high energy consumption because the hot rollers must rotate at a high speed.
Japanese Examined Patent Publication (Kokoku) No. 35-3104 discloses that a fiber durable for practical use can be obtained by taking up a yarn spun from a spinneret at a high speed. This method, however, requires an expensive, high speed winder, and in addition, it is difficult to maintain a stable operation by avoiding the many filament breakages occurring in the yarn thus produced.
Japanese Examined Patent Publication (Kokoku) No. 45-1932 proposed a method for producing a drawn yarn, comprising the steps of melt-spinning a thermoplastic polymer, cooling the yarn thus obtained, running the yarn through a hot zone maintained at a temperature above 80.degree. C., and taking up the heat-treated yarn at a speed higher than 4000 m/min. Although a yarn can be produced at a lower cost according to this method, the mechanical properties thereof are still inferior to those of the conventional drawn yarn. In addition, a high speed take-up at above 4000 m/min causes similar defects to those of the above prior art procedures.
A method for obtaining a drawn yarn by running an as-spun yarn through a liquid bath is proposed, for example, in Japanese Examined Patent Publication (Kokoku) No. 35-2721 (corresponding to U.K. Patent No. 803237), Japanese Examined Patent Publication (Kokoku) No. 38-2016 (corresponding to U.K. Patent No. 828986) and Japanese Unexamined Patent Publication (Kokai) No. 58-169513. According to this method, due to a viscous resistance of liquid bath the yarn can be drawn to form a drawn yarn with less elongation. The strength of the yarn, however, is reduced due to a shock exerted thereon when drawn by the resistance of the liquid bath, and further the yarn has an inferior thermal stability, i.e., has a high thermal shrinkage rate in both its dry and wet conditions.